Perpetual Inventory:

A system that maintains an expected inventory level within a store that reflects all physical product movement sales, deliveries, credits, etc. is called Perpetual Inventory.
In business and accounting/accountancy, perpetual inventory or continuous inventory describes systems of inventory where information on inventory quantity and availability is updated on a continuous basis as a function of doing business. Generally this is accomplished by connecting the inventory system with order entry and in retail the point of sale system. In this case, book inventory would be exactly the same as, or almost the same, as the real inventory.
In earlier periods, non-continuous or periodic inventory systems were more prevalent. Starting in the 1970's digital computers made possible the ability to implement a perpetual inventory system. This has been facilitated by bar coding and lately radio frequency identification (RFID) labeling which allows computer systems to quickly read and process inventory information as part of transaction processing.